Such a drift chamber is for example known from U.S. Pat. No. 4,777,363.
Ion mobility spectrometers (IMS) are generally used to detect the presence of materials in a surrounding, e.g. of harmful substances in the atmosphere. The possibilities extend from a pure alarm function in the presence of a known harmful substance, e.g. a chemical warfare agent, via the identification of an unknown substance to a quantitative determination of the concentration.
Typical IMS comprise an ionization source, a reaction cell, a drift chamber, e.g. in the form of a tube, an entrance grid between the reaction region and the drift region, and an ion detector. The spectrometers at atmospheric pressure, where the average path length of the gas molecules within the drift chamber is small compared to the chamber dimensions. Usually, a carrier gas, generally dry air, is introduced into the spectrometer together with the sample gas or vapour. The carrier gas which contains the sample is fed via an inlet onto the ionization source, resulting in a partial ionization of the molecules of the carrier gas and the sample. The ionization source consists generally of tritium or .sup.63 Ni. Additional charges are transferred by means of impact from molecules of the carrier gas to sample molecules, or quasi-molecule ions are formed. Within the reaction region usually an electrical potential gradient is present, so that the charged mixture is moved towards the injection grid. The grid is charged electrically and blocks the transfer to the drift chamber under normal conditions. However, for short times this potential is periodically reduced, so that a number of sample ions reaches the drift chamber in a pulse-like manner. Here an approximately constant electrical drift field, i.e. a constant potential gradient, is present, which moves the ions along the cell axis towards a detector electrode, which is located at the end of the drift chamber opposite to the injection grid and which collects the charge of the ions. The time of arrival of the ions with respect to the pulse-like opening of the injection grid depends on the mobility of the detected ions. Light ions are more mobile than heavy ones and reach the detector earlier. This effect is used to characterize the ions. The pulse-like opening of the grid can be repeated periodically to increase the signal-to-noise-ratio or to perform a quasi-contenuous measurement.
The drift chamber of the known spectrometer is part of a cylindrically shaped tube with an injection grid said grid separating a reaction region containing a .sup.63 Ni source from the drift region. The actual drift tube comprises a multitude of metal rings ranging from the injection grid to the detector electrode at the opposite end of the drift region and which are separated by insulating rings. The stack of metal and insulating rings is compressed along the axis and fixed by axial rods. Via a resistor network electrical potentials may be applied to the injection grid and to the metal rings from a 3000 volt high voltage source, resulting in an axial electrical field of 221 volts per centimeter inside the drift chamber. In alternatively described embodiments of the cell, this cell is hermetically sealed from the outside by O-rings between the metal rings or the rings are arranged outside a tube made from glass or "TEFLON". In a further embodiment the electronics to drive the spectrometer is arranged on a flexible printed circuit which is wrapped around the drift chamber.
GB-A1 2 217 103 discloses electrode rings in the form of copper bands which are glued onto the outside of an IMS drift chamber made from a non-conductive material, particularly glass.
In U.S. Pat. No. 4,390,784 a drift chamber made from ceramics or glass is described which is continuously coated on its inside with a resistive coating, to produce the constant axial electrical field.
U.S. Pat. No. 4,633,083 discloses an IMS drift chamber with a multitude of stainless steel electrode rings with a T-shaped profile, which are separated by insulating glass rings.
In particular with regard to an IMS for mobile, e.g. military, application as a series product, despite known spectrometers, there is still a need for a robust, reliable apparatus with a gas-tight drift chamber which is simply and cost effectively produced.